The United States Government has certain rights in this invention. This invention was made under a CRADA (CRADA No. BG-00-441) between American Superconductor Corporation and Lawrence Berkeley National Laboratory operated for the United States Department of Energy.
1. Field of the Invention
This invention relates to epitaxial crystal growth on the surface or in the interior of a substrate. More particularly, this invention relates to a process for the formation of a biaxially ordered layer on the surface of a non-single-crystal substrate to provide a surface which permits subsequent epitaxial growth of a biaxially oriented crystalline film thereover or therein.
2. Description of the Related Art
Traditionally, high temperature superconducting thin films were grown on single crystal substrates which promote the growth of oriented epitaxial films, and the resultant structures were suitable for a limited number of electronic applications. However, such single crystal substrates are not suitable for conductor applications such as electric power transmission, magnetic energy storage, motors, or the like.
To form superconducting thin films for a greater number of conductor applications, metal substrates are typically used. Unfortunately, the metal substrate does not have the desired degree of biaxial orientation of the superconducting film as obtainable with single crystal substrates. In attempting to establish biaxial orientation and avoid metal migration from the substrate into the superconducting film (which can destroy the film""s superconducting properties) an intermediate layer is usually formed over the metal substrate before depositing the superconducting film.
Several approaches have been used to promote biaxially oriented crystalline growth on substrates that do not provide an epitaxial template. In one approach, improved superconducting film orientation is attempted by depositing a buffer layer of yttria-stabilized zirconia (YSZ) or MgO using vapor deposition at an inclined angle. However, the deposited layers have a large degree of tilt towards the axis of the vapor source (xcx9c25xc2x0), and this method requires deposition of a thick intermediary layer ( greater than 1 xcexcm) of YSZ or MgO to attain the desired degree of biaxial orientation.
Another approach for forming oriented superconductor films utilizes metallographic rolling and thermal annealing to induce biaxial orientation directly in a metal foil such as Ni metal foil. Difficulties with oxidation of the metal surface during deposition and problems transferring the epitaxial template to the superconducting film require a multilayer buffer structure between the superconductor and the substrate, resulting in increased manufacturing costs. Further, this method is limited to only a few metals, and is therefore not generally useful in forming near-single-crystal thin films using a variety substrate materials.
Another approach for fabricating superconductor tapes on flexible metal foil is ion-beam assisted deposition (IBAD) of an oriented template layer. The IBAD process utilizes oblique angle ion bombardment during the deposition of a intermediate layer, most commonly YSZ or MgO, to produce a biaxially aligned template layer. The advantage of this process is its ability to form a template layer on nearly any substrate, permitting use of a wide variety of near-single-crystal thin films on substrates that do not provide a template for epitaxial crystalline growth. However, in the case of YSZ, results have shown that the texture of the IBAD YSZ buffer layer improves with thickness, and therefore deposition time. To produce the texture necessary for superconducting tapes, thick YSZ films are needed. Since IBAD deposition rates of YSZ are typically very slow, deposition times are often too slow for practical applications.
In our previous U.S. Pat. No. 5,432,151, we disclosed an IBAD process for simultaneous deposition and orientation of a biaxially textured layer on a substrate using laser ablation to deposit the biaxially orientable material and an oblique ion beam to biaxially orient the material as it is deposited. However, it would be advantageous to provide independent control of the deposition process and the biaxial orientation process so that a material may be biaxially oriented without regard to the manner in which the biaxially orientable material was formed (e.g., deposited or grown) on an underlying substrate.
Extending beyond superconducting films, there are an increasing number of methods which include deposition of near-single-crystal quality thin films on substrates that do not provide a template for epitaxial crystalline growth. These substrates include many technically important materials such as randomly-oriented polycrystalline metal foils, amorphous insulators such as SiO2, and plastics.
It would, therefore, be desirable to provide a process for forming a biaxially oriented surface on a variety of substrates, from which surface an epitaxial crystalline formation can readily be grown. The present invention achieves this goal and provides additional advantages as well.
The invention provides a method of increasing the extent of a desired biaxial orientation of a previously formed non-single-crystal structure by contacting said structure with an oblique particle beam thereby forming in the structure a nucleating surface having increased desired biaxial orientation. In one embodiment, the method further includes a step of depositing a layer onto the previously formed structure, where the layer is capable of attaining a biaxial orientation in registry with said nucleating surface. In another embodiment, the invention further includes a step of epitaxially growing the crystalline formation using the nucleating surface to promote the epitaxial growth.
The invention further provides an at least partially crystalline structure containing a nucleating surface formed by contacting a previously formed non-single-crystal structure with an oblique particle beam, and a crystalline active layer. This structure further contains 0 to 10 orientation-transmitting layers adjacent and between the nucleating surface and the active layer, where the active layer is oriented in registry with the nucleating surface.